The present invention relates to a method and a system of reading a radiation image that has been stored in a photostimulable phosphor screen, wherein the photostimulable phosphor screen can be re-used. The invention further relates to a re-usable radiation detector.
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In such a system it is preferred, in view of economy, that the stimulable phosphor screen can be used in many imaging cycles.
The reuse of the stimulable phosphor screen is possible when the previously stored radiation image is erased to a sufficient extent.
When reading out an image by scanning a phosphor screen that has been exposed to penetrating radiation, less than 90% of the stored energy is released. Thus there arises a problem that, upon reuse, part of the radiation image is still stored in the phosphor screen and can appear in the subsequent image as a so-called ghost image.
In general medical radiography, images are made with widely differing X-ray doses.
To make images of extremities, like e.g. fingers, doses are used of the order of 1 mR. On the other hand, images of internal organs, like the stomach are made with X-ray doses that may be as high as 300 mR.
To avoid ghosting, when making a 1 mR image immediately after a 300 mR image, the signal of the first image must be reduced by more than a factor of 300.
As a matter of fact, a dynamic range is desired in the second image of at least 100. This implies that the signal created by the first irradiation must be reduced by a factor of at least 3.104, which is equivalent to requiring an erasure depth of 1/(3.104)=3.3.10xe2x88x925.
According to U.S. Pat. No. 3,859,527 (column 4, lines 5-7) the phosphor can be reduced to neutral state by actions like a uniform illumination, irradiation or heating.
In commercial systems, the phosphor screen is erased by illumination with visible light.
Commonly incandescent lamps are used because they are cheap, high power light sources. High power light sources are selected, because in order to guarantee a high through-put scanning system, the phosphor screen must be erased in a short time. High power lamps, however, generate a lot of heat, which may destabilise the scanner to read out the storage phosphor screens. In order to sufficiently remove the heat generated by the high power lamps the size of the erasing unit has to be rather large.
Furthermore, in case incandescent lamps are used in an erasure unit of a phosphor read out apparatus, the dimensions of the phosphor erasure unit are determined by the dimensions (more specifically the diameter) of the incandescent lamps.
It is an object of the present invention to provide a method and a system for reading a radiation image on a photostimulable phosphor screen wherein the screen is erased in between successive recordings to an adequate extent so as to permit re-use of the screen.
It is a further object of the present invention to provide such a system that is compact and has at the same time a high throughput.
Still another object is to provide a compact re-usable radiation detector.
Further objects will bercome apparent from the description given below.
The inventors have found that the above mentioned objects are realised by a method of reading a radiation image that has been stored in a photostimulable phosphor screen comprising the steps of
(1) stimulating said phosphor screen by means of stimulating radiation,
(2) detecting light emitted by the phosphor screen upon stimulation and converting the detected light into a signal representation of said radiation image,
(3) erasing said phosphor screen by exposing it to erasing light, wherein
(4) said phosphor is a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide, and
(5) said erasing light is emitted by at least one electroluminescent lamp.
In this document the term xe2x80x9cradiationxe2x80x9d has to be understood as any penetrating radiation and includes irradiation originating from a radioisotope (e.g. Co60, Ir192, Se75, etc.), radiation created by an X-ray generator of any type, radiation and high energy particles created by a high energy radiation generator (e.g. Betatron), radiation from a sample labelled with a radioisotope as is the case in e.g. autoradiography.
An electroluminescent lamp can be in the form of an electroluminescent film based on an inorganic electroluminescent phosphor, e.g. ZnS:Mn, or an electroluminescent film based on organic light-emitting diodes (OELDs).
The use of electroluminescent lamps in the present invention is advantageous in that these lamps are ideal for uniform illumination applications.
While eliminating the need for sockets, bulbs, diffusers and reflectors, these lamps provide uniform lighting across the entire lamp surface. The lamps are a cold light source, so little heat is added to the assembly.
Most light emitting devices vary in luminance according to the direction. Electroluminescent lamps have essentially the same luminance independent of angle, i.e. an electroluminescent lamp is a Lambertian emitter.
Moreover, electroluminesent lamps can be made with an emitting surface that is of the same size as the surface of the phosphor screen that must be erased. This makes it possible to erase phosphor screens in a very homogeneous way.
Nevertheless a prejudice exists against the use of electroluminescent lamps as erasing light source in a photostimulable phosphor read out system because the power of the light source is low and consequentially these lamps are thought to be inadequate for obtaining a sufficient erasure depth so as to enable re-use of the photostimulable phosphor screen.
The inventors have found that by using a specific phosphor, more specifically a divalent europium activated caesium halide phosphor, wherein said halide is at least one of chloride and bromide, erasure to a sufficient extent can be obtained with a low power and compact electroluminescent lamp. In this way the erasure device can be made very compact without implying a longer erasure time and consequentially a lower throughput.
Another aspect of the present invention relates to a radiation image read out apparatus as set out in claims 8 to 12.
The compactness of an erasure unit which comprises an electroluminescent lamp makes it appropriate for integration in a radiation detector according to the present invention.
Still another aspect thus relates to a radiation detector as set out in claim 13 and following claims.
Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
In a first embodiment of the method, of the system and of the detector according to the present invention (an) electroluminescent lamp(s) is(are) used that is(are) based on inorganic electroluminescent phosphors as e.g. ZnS:Mn, ZnS:Cu, CaS:Eu, CaS:Ce.
These electroluminescent lamps can have an optical power of upto ca. 0.3 mW/cm2.
Furthermore, by adjusting the phosphor composition and the operating frequency of the lamp, the light spectrum emitted by the electroluminescent lamp can be matched with the erasure spectrum of the phosphor. Hence, electroluminescent lamps lead to higher erasure depth as incandescent lamps at equal optical power while having much smaller dimensions and dissipating less heat.
It will further be explained that an optical energy of 10 mJ/cm2 is needed to erase a CsBr:Eu2+ phosphor to a sufficient extent. This implies that the CsBr:Eu2+ phosphor can be erased with an electroluminescent lamp in about 35 s.
When compared with the energy needed to erase a commercially available BaFBr:Eu2+ phosphor screen, denoted by the trade name MD-10 of Agfa-Gevaert N.V., at least 100 times more energy would be needed for erasure. This would corresponds to an erasure time of ca. 1 hour. An erasure process of this duration would be unacceptable for commercial purposes.
The use of an electroluminescent lamp in combination with a CsBr:Eu2+ phosphor is furthermore advantageous in that this combination provides very homogeneous erasure in a compact erasure unit.
When compared with a situation wherein a BaFBr:Eu2+ phosphor would have been used, a similar result could only be obtained with very long erasure times, which is highly impractical.
Erasure of the CsBr:Eu2+ phosphor screen can take place in a compact erasure unit in the digitiser. The electroluminescent lamps being very flat, however, erasure can also take place in the cassette, or in a box, in which the storage phosphor screens are stored. In this way a re-usable radiation detector can be made.
In a second embodiment an organic electroluminescent lamp (OELD) is used. Although this type of electroluminescent lamps is more expensive, it is even more suited for use in a storage phosphor erasure unit because this type of lamps allows even shorter erasure times as will be explained below.
Organic electroluminescent devices are either based on vacuum-evaporated small organic molecules or on polymers. The former class of organic electroluminescent lamps are usually called OELD, the latter Polymer ELD.
The most simple organic electroluminescent lamp structure consists of a single layer between two suitable contacts: for the hole-injection anode ITO (Indium Tin Oxide) is frequently used while for the electron-injection cathode a Mgxe2x80x94Ag alloy, Al or Ca is used.
The operation is as follows: in organic electroluminescent lamps electrons and holes are injected into a rather well insulating layer, where they recombine via a process that leads to the emission of a photon.
The organic electroluminescent lamp has a very thin layer (about 100 nm) and the voltage is low ( less than 10V).
For a voltage of ca. 10 V, organic electroluminescent devices reach an efficiency of ca. 2 mW/cm2. This means that the optical power of these devices is an order of magnitude higher than the optical power of inorganic electroluminescent lamps. With organic electroluminescent devices the erasure time can therefore be reduced by an order of magnitude, which brings the erasure time down to ca. 5 s. This duration is adapted for use in actual commercial scanners for storage phosphor screens.
Another aspect of this invention relates to the use of light emitting diodes (LED) as erasing light source(s) in a photostimulable phosphor read out method, system and detector as set out in claims 4, 9 and 14.
Arrays of light emitting diodes are almost equally compact as electroluminescent lamps, in addition they have a higher optical power. This implies that they are very suitable for the construction of a compact erasure unit that will erase the phosphor plate very homogeneously.
By using a specific phosphor, more specifically a divalent europium activated cesium halide phosphor, wherein said halide is at least one of chloride and bromide, erasure to a sufficient extent can be obtained with a low power and compact light emitting diodes within a short period of time. In this way the erasure unit can be made very compact without implying a longer erasure time and consequentially a lower throughput.
Light emitting diodes with a size of 3 mmxc3x973 mm may have a luminance of upto 500 mCd. Arrays of such light emitting diodes will have an optical power of 5 Cd/m2. This corresponds to ca. 50 mW/cm2.
An erasure unit, made up of an LED array with an emitting area of ca. 60 cm2 can be used, therefore, in order to erase a CsBr:Eu2+ in 5 s. Alternatively an LED array with an emitting area of 300 cm2 can be used to erase a CsBr: Eu2+ screen of 43 cmxc3x9735 cm in 1 s.
The present invention as well as specific and/or preferred embodiments hereof will be explained in the detailed description given below. Particular aspects will be illustrated by the drawings enumerated hereinafter.